Manufacture of coiled coil or double helical filaments



Patented Mar. 17, 1936 UNITED STATES M A m JFACTURE or 'COILED con. BDOUBLE HELICAL FILAll/[ENTS Colin James 'Smithells, Middlesex, England,assi'gnor to General Electric Company, a corporation of New York NoDrawing. Application July 1, 1935,1Seriail No. 29,371. In Great BritainJuly 18, 1934 4 Claims.

My invention relates to the manufacture of coiled coil or double helicalfilaments for electric incandescent lamps, that is, filaments in themanufacture of which a straight wire is coiled 5 into a primary helix ona mandrel and this primary helix with its mandrel is then coiled into asecondary helix. It is to be understood that the filaments referred toherein are tungsten.

One of the main problems in the manufacture of such filaments is toprevent distortion of the helices when heated in the finished lamp. Ithas long been known that this problem can be solved by a stabilizingprocess, that is, by heating the coiled coil, before the mandrels areremoved, to

a temperature at which the stresses set up during Winding are relieved.However, if the heat ing is indefinitely prolonged and is carried out atsuch a high temperature that complete recrystallization of the metaloccurs, the filament becomes brittle and liable to break when handled.This temperature is generally believed to be in the neighborhood of 1500C.

It appears that this problem might be avoided by heating the filament toa temperature so low that complete recrystallization cannot occur. It

has been proposed already to heat coiled coil filaments to a temperaturebelow that of recrystallization, and in particular to 1300 C., for aboutan hour.

I have found that this problem can also be avoided by heating thefilament to a considerably higher temperature for a much shorter period.In fact by choosing the proper temperature for the furnace, the desiredstabilization can be effected, without causing brittleness, by drawingthe wire continuously through the furnace at a speed of the order of onemeter per minute, and so that the wire remains in the furnace for aperiod much less than one minute.

It is believed that the effect of such a brief treatment at a relativelyhigh temperature is to produce complete or nearly completerecrystallization in the outer layers of the wire, while leaving thecore substantially unchanged. The

strains produced by coiling are concentrated mainly on the outer layers,and the recrystallization of this part will relieve most of the strain.The speed of recrystallization increases with the amount of strain, sothat the core will not recrystallize and will provide the necessarystrength.

According to the invention, the manufacture of coiled coil filaments inwhich tungsten wire, first wound on a primary mandrel, is then woundtogether with its primary mandrel on a continuous length of secondarymandrel, comprises the step of drawing the coil on its mandrelscontinuously through a furnace, the temperature of the furnace being sohigh that if the wire were maintained at this temperature for one hourit 5 would recrystallize completely, and the duration of its stay in thefurnace being so short that recrystallization is not complete. Atemperature between 1500 C. and 1600 C. and a duration of the order oftwenty seconds are generally 10 suitable. The extent ofrecrystallization is, of course, to be judged with reference tostability and brittleness. Recrystallization begins when stabilitybegins to increase and is complete when further maintenance at the sametemperature 15 produces no further increase either in stability or inbrittleness.

The temperature of the furnace should be as uniform as possible alongits length, but perfect uniformity is never attained. It is to be under-20 stood therefore that the value of 1500 C. to 1600 C. refers to themaximum temperature as determined by the ordinary factory methods, andthat in determining the period during which a point on the filamentremains within the furnace, any 25 regions where the temperature is lessthan 1000 C. are to be regarded as outside the furnace.

In a specific example of the process according to the invention, astraight tungsten wire is coiled into a helix on a primary mandrel ofmolybdenum. 30 The coiling is continuous. This primary coil and mandrelis then coiled on a secondary mandrel, also of molybdenum, uncoiled gapsbeing left at which the secondary helix can be cut to provide theindividual filaments. The structure so formed 35 is then drawncontinuously through a molybdenum tube of ten mm. internal diameter andthirty-two cm. long, in a hydrogen-nitrogen atmosphere, the tube beingheated to a temperature of 1550 C. The coil on its mandrels is drawn 40through at a rate of one and one-half meters per minute. The temperatureis determined by viewing the tube with an optical pyrometer. The lengthof the fully heated portion of the furnace is about thirty cm., so thatthe coil remains in 45 this portion about twelve seconds. The mandrelsare then removed by solution in the ordinary manner. The filaments inthis particular case are intended for or watt 230 volt lamps. If finerfilaments are being made, such as are used 50 for 40 watt 230 voltlamps, the temperature of the furnace may be reduced to 1500 C., thespeed of travel being the same.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: 55

1. The method of manufacturing coiled coil filaments for electricincandescent lamps in which tungsten Wire is first coiled on a primarymandrel and then together with its mandrel is coiled on a secondarymandrel, comprising the step of drawing the coiled coil on its mandrelsthrough a furnace at a temperature of approximately 1500 C. to 1600 C.at a speed such that the wire is not completely recrystallized.

2. The method of manufacturing coiled coil filaments for electricincandescent lamps in which tungsten wire is first coiled on a primarymandrel and then together with its mandrel is coiled on a secondarymandrel, comprising the step of drawing the coiled coil on its mandrelsthrough a furnace at a temperature of approximately 1500 C. to 1600 C.at a speed such that a given portion of the wire remains in the furnacefor less than one minute.

3. The method of manufacturing coiled coil filaments for electricincandescent lamps in which tungsten wire is first coiled on a primarymandrel and then together with its mandrel is coiled on a secondarymandrel, comprising the step of drawing the coiled coil on its mandrelsthrough a furnace at a temperature of approximately 1500 C. to 1600 C.at a speed such that a given portion of the wire remains in the furnacebetween five and sixty seconds.

4. The method of manufacturing coiled coil filaments for electricincandescent lamps in which tungsten wire is first coiled on a primarymandrel and then together with its mandrel is coiled on a secondarymandrel, comprising the step of drawing the coiled coil on its mandrelsthrough a furnace at a temperature of approximately 1500 C. to 1600 C.in a reducing atmosphere at a speed such that a given portion of thewire remains in the furnace for less than one minute.

COLIN JAMES SMITHELLS.

